Direct-view and projection display systems conventionally employ kinescopes as display devices. In normal operation of a kinescope an electron beam is deflected by scanning circuitry to produce a relatively large area raster on the face plate of the kinescope and video modulation of the beam produces a visible picture by activating phosphors deposited on the face plate. The beam energy in normal operation is distributed over the whole area of the kinescope face plate. If scan loss should occur, this energy may be concentrated to a relatively small area and this high concentration of energy may produce permanent damage to the phosphor, a so-called "spot burn". Scan loss may occur during turn-on under so-called "hot start" conditions, it may also occur during turn-off and it may also occur during normal operation of the display due, for example, to a component failure.
It is generally known to protect against scan loss by detecting or "predicting" the scan loss occurrence and in response to the detected or predicted occurrence applying grid-to-cathode bias to the kinescope of a value sufficient to blank or cut-off the electron beam. Three examples of beam cut-off for spot-burn protection are discussed below.
In a first example of beam cut-off for spot burn protection, the beam cut-off is provided by driving the kinescope cathode driver amplifier to its maximum positive output voltage corresponding to peak black level ("blacker than black"). This technique is exemplified by U.S. Pat. No. 4,660,093 entitled TELEVISION RECIEVER WITH DELAYED DISPLAY which issued to Craig et al. on Apr. 21, 1987. In this example a control circuit is included for inhibiting the operation of the kinescope driver stages for a short interval after the receiver is energized, particularly under "hot start" conditions, to prevent disturbing artifacts from being displayed by the kinescope. The circuit drives the kinescope cathodes to cut-off ("blacker than black" level) by clamping the base of the output transistor of a cascode driver amplifier to ground.
In a second example of spot burn protection, beam cut-off is provided by driving the control grid negative relative to its normal bias value. This techniques is exemplified by U.S. Pat. No. 4,340,910 entitled CRT SPOT SUPPRESSION CIRCUIT which issued to Valdes on 20 July 1982. In this circuit, a scan indicating signal is applied via a parallel resistor and capacitor to the anode of a PN diode and to the control grid of the kinescope. The cathode of the diode is grounded. In operation, the scan indicating signal charges the capacitor and a portion of the signal flows through the resistor to develop a grid bias voltage across the diode of about 0.6 volts. Upon scan loss the reduction in scan indicating voltage is coupled via the charged capacitor to the diode thereby reverse biasing the diode and driving the kinescope grid negative to achieve grid cut-off thereby cutting off the electron beam of the kinescope.
In a third example of spot burn protection, scan loss is predicted and the control grid is driven negative prior to the actual occurrence of scan loss. This protection technique is exemplified by U.S. Pat. No. 4,488,181 entitled ELECTRON BEAM SUPPRESSION CIRCUIT FOR A TELEVISION RECEIVER which issued to Haferl on Dec. 11, 1984. In this example, a remote control circuit develops an on/off command signal for switching between normal and standby modes of operation. A first switch is responsive to the command signal and disables normal generation of the scanning current upon the occurrence of the off-state of the command signal to inhibit normal sweep of the electron beam. A second switch, also responsive to the command signal, applies to one of the electrodes of the picture tube, such as the grid electrode, a blocking potential upon the occurrence of the off-state of the command signal to suppress generation of the electron beam prior to the disablement of the scanning current generation. In a specific embodiment of the invention, the blocking potential is provided by a circuit comprising a capacitor having a first plate connected to a charging source and having a second plate connected to the kinescope grid and coupled to ground via a PN diode. In normal operation the capacitor is charged by the charging source and a resistor in parallel with the capacitor applies forward bias to the diode thereby establishing a grid bias voltage for the kinescope of about 0.6 volts. Grid blocking is provided by a clamp transistor which clamps the first plate of the capacitor to ground in response to the turn-off command thereby reverse biasing the diode and driving the grid to a negative potential.